Projection optical system and image display apparatus

ABSTRACT

A projection optical system for use in an image display apparatus having an illumination optical system applying light from a light source, and an image display device receiving the light from the illumination optical system to form a projection image includes a projector lens composed of plural lenses, a first mirror, and a second mirror formed of a concave mirror. The projection optical system is configured to project the projection image onto a projection surface. A projection luminous flux passing through the projector lens to be incident on the first mirror is a luminous flux exhibiting divergence. The projection luminous flux reflected off the second mirror after having reflected off the first mirror is converged once, and the once converged projection luminous flux is projected onto the projection surface. A lens surface of a lens located closest to the first mirror among the lenses of the projector lens is convex.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. application Ser. No.13/611,578, filed Sep. 12, 2012, and is based upon and claims thebenefit of priority from Japanese Patent Application No. 2011-202691filed on Sep. 16, 2011, and Japanese Patent Application No. 2011-223983filed on Oct. 11, 2011, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosures discussed herein relate to a projection optical systemand an image display apparatus having the projection optical systemcapable of enlarging an image to display the enlarged image on a screen.

2. Description of the Related Art

There is an image display apparatus having a projection optical systemknown in the art, which is capable of being placed at a relativelycloser position to a screen than a position at which the related artimage display apparatus is allowed to be placed. Such an image displayapparatus is called a “close range projector”. The purposes of the closerange projector being devised are as follows. First, the projector willnot project light too bright to blind a presenter or a demonstrator whostands close to the screen, and secondly, the projector will not emit anexhaust gas or noise to adversely affect the audience who watches andlistens to the presentation.

Such a close range projector may utilize a related-art projectionoptical system (coaxial and rotationally symmetric) to reduce a distancebetween the projector and a screen surface by widening the angle of viewof the projection optical system, or may utilize a curved mirror. Thus,the close range projector having the related-art projection opticalsystem capable of widening the angle of view may achieve theaforementioned purposes by improving the related-art technology.However, in the above related-art projection optical system, an outerdiameter of a lens arranged close to the screen side may need to beincreased, which may result in an increase in the size of the projectoritself. By contrast, the close range projector having the curved mirrormay be capable of projecting light at an extremely close range withoutincreasing the size of the projector itself.

Examples of the close range projector utilizing the curved mirror aredisclosed in Japanese Patent No. 4329863 (hereinafter called “PatentDocument 1”) and Japanese Patent No. 3727543 (hereinafter called “PatentDocument 2”). In the close range projector disclosed in Patent Document1, a concave mirror is arranged behind the lens optical system forprojecting light. In the close range projector disclosed in PatentDocument 2, a convex mirror is arranged behind the lens optical systemfor projecting light. In either cases, arrangement accuracy between thecomponents may be improved by simply arranging lenses and a mirrorsequentially. However, in both cases, a long distance may be requiredbetween the lens optical system and the mirror, which may result in anincrease in the size of the projection optical system.

Meanwhile, examples of the close range projector capable of reducing adistance between the lens optical system and the mirror are disclosed inJapanese Laid-open Patent Publication No. 2009-157223 (hereinaftercalled “Patent Document 3”) and Japanese Laid-open Patent PublicationNo. 2009-145672 (hereinafter called “Patent Document 4”). These closerange projectors disclosed in Patent Document 3 and Patent Document 4include a reflector configured to bend or fold an optical path having along distance between the lens optical system and the reflector, whichmay reduce the size of the projection optical system.

In the projector disclosed in Patent Document 3, the size of theprojection optical system may be reduced by sequentially arranging aconcave mirror and a convex mirror and a convex mirror subsequent to thelens optical system. In the projector disclosed in Patent Document 4,the size of the projection optical system may be reduced by arranging aplane mirror behind a concave mirror.

However, either of the projection optical systems disclosed in PatentDocuments 3 and 4 have a long distance between an image display deviceand a curved mirror. Hence, if a user desires to place the projectormain body further at a position closer to the screen than allowablepositions at which the related art projectors are placed, a length ofthe projection optical system itself may become an obstacle.

Japanese Patent No. 4210314 (hereinafter called “Patent Document 5”)discloses a technology for resolving such a limitation of “the size ofthe projection optical system itself”. More specifically, PatentDocument 5 discloses a projection optical system having an image displaydevice, a display surface of which is orthogonal to a screen surface.With this vertical configuration, the projector main body may be capableof being placed even closer to the screen than the allowable positionsof the related art projectors because the obstacle due to the length ofthe projection optical system itself may be resolved or eliminated.

However, although the projection optical system has the verticalconfiguration such as the one disclosed in Patent Document 5, which iscapable of projecting an image at an extremely close range whilereducing its size, divergence of light incident upon a mirror systemfrom the lens optical system may need to be increased in order for theprojector placed closer to the screen to display a larger image on thescreen projection. However, if divergence of light is increased, thefollowing three problems may occur.

That is, first, it may become difficult to correct aberration ofprojection luminous flux passing through parts other than an opticalaxis of the lens system. Second, the diffused luminous flux strikes alens surface closest to the first mirror before striking the concavemirror. Third, the light reflected off the concave mirror strikes thefirst mirror in the middle of a route toward the screen before reachingthe screen.

As illustrated in the technology disclosed in Patent Document 5, if alens surface closest to the first mirror is concave, each of lenssurfaces is gradually protruded toward the first mirror side with theincreasing distance from an optical axis to the position of acorresponding lens. Further, in view of aberration correction and errorsensitivity, when a lens surface closest to the mirror is concave, therefracting angle of the beam is extremely large in the projectionoptical system that manages the luminous flux having increaseddivergence. Hence, it may become difficult to correct aberration in anentire region of the screen. In this case, since the refracting angle ofthe beam is large, image quality may drastically deteriorate even if theslightest shifts are present in the arrangement of the components of theprojection optical system.

Further, in the vertical projection optical system disclosed in PatentDocument 5 having a projector lens arranged parallel to the screen,particles such as dust may be easily be attached to the projector lensor mirrors compared to the projector lens in a horizontal projectionoptical system having a projection optical system arranged orthogonal tothe screen. In addition, in the configuration of the above case,attached dust may fall onto the vertical projection optical system.Accordingly, if particles or dust attached to the projection systemremain attached, the attached particles may be displayed on the screenwithout automatically coming off from the projector lens due to thegravitational effect.

Moreover, since the angle of view of the lens optical system is narrowin the projection optical system disclosed in Patent Document 5, theparticles attached to one of the two mirrors, each of which causes theprojection luminous flux to reflect off its surface, may affect theamount of light projected onto a projection surface of the screen. Inthis case, even if a dustproof glass is arranged between the mirror andthe projection surface of the screen on which the projection light isprojected, it may be difficult to prevent fine particles or dust havinga size of 0.01 mm or less from intruding onto the projection surface.

RELATED ART DOCUMENT Patent Document

-   Patent Document 1: Japanese Patent No. 4329863-   Patent Document 2: Japanese Patent No. 3727543-   Patent Document 3: Japanese Laid-open Patent Publication No.    2009-157223-   Patent Document 4: Japanese Laid-open Patent Publication No.    2009-145672-   Patent Document 5: Japanese Patent No. 4210314

SUMMARY OF THE INVENTION

Accordingly, it is a general object of an embodiment of the presentinvention to provide a projection optical system and an image displayapparatus having the projection optical system capable of projecting anenlarged image on a screen when an image is projected from a positioncloser to a projection surface of the screen.

Further, it is an additional object of the embodiment of the presentinvention to provide a projection optical system and an image displayapparatus having a vertical projection optical system capable ofpreventing an adverse effect on the amount of light projected onto aprojection surface due to particles such as dust.

According to one embodiment, there is provided a projection opticalsystem for use in an image display apparatus having an illuminationoptical system configured to apply light emitted from a light source,and an image display device configured to receive the light applied fromthe illumination optical system to form a projection image. Theprojection optical system includes a projector lens composed of aplurality of lenses; a first mirror; and a second mirror formed of aconcave mirror, the projection optical system being configured toproject the projection image formed by the image display device onto aprojection surface. In the projection optical system, a projectionluminous flux passing through the projector lens to be incident on thefirst mirror is a luminous flux exhibiting divergence. Further, in theprojection optical system, the projection luminous flux reflected offthe second mirror after having reflected off the first mirror isconverged once, and the once converged projection luminous flux is thenprojected onto the projection surface. Further, in the projectionoptical system, a lens surface of a lens located closest to the firstmirror among the lenses of the projector lens is convex.

According to another embodiment, there is provided a projection opticalsystem for use in an image display apparatus having an illuminationoptical system configured to apply light emitted from a light source,and an image display device configured to receive the light applied fromthe illumination optical system to form a projection image. Theprojection optical system includes a lens optical system composed of aplurality of lens groups; and a mirror optical system composed of afirst mirror, and a second mirror formed of a concave mirror, theprojection optical system being configured to project the projectionimage formed by the image display device onto a projection surface. Inthe projection optical system, an intermediate image is formed betweenthe first mirror and the second mirror, the intermediate image beingcomposed of pixels associated with the image display device locatedclosest to an optical axis of the lens optical system, and a lenssurface of a lens group located closest to the first mirror among thelens groups of the lens optical system is convex.

According to another embodiment, there is provided an image displayapparatus that includes one of the above projection optical systems; anillumination optical system configured to apply light emitted from alight source; and an image display device configured to receive thelight applied from the illumination optical system to form a projectionimage.

Additional objects and advantages of the embodiments will be set forthin part in the description which follows, and in part will be obviousfrom the description, or may be learned by practice of the invention.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and further features of embodiments will be apparent fromthe following detailed description when read in conjunction with theaccompanying drawings, in which:

FIG. 1 is an optical layout schematically illustrating an example of animage display apparatus according to an embodiment;

FIG. 2 is a side view illustrating an example of a main part of aprojection optical system provided in an image display apparatusaccording to an embodiment;

FIG. 3 is a ray diagram illustrating features of light projected by theprojection optical system;

FIG. 4 is a side view illustrating an example of a main part of aprojection optical system provided in the image display apparatusaccording to the embodiment;

FIG. 5 is a ray diagram illustrating features of light projected by theprojection optical system;

FIG. 6 is a side view illustrating another example of a main part of aprojection optical system provided in an image display apparatusaccording to another embodiment;

FIG. 7 is a side view illustrating an example of a main part of aprojection optical system provided in an image display apparatusaccording to still another embodiment;

FIG. 8 is a side view illustrating an example of a main part of aprojection optical system provided in an image display apparatusaccording to still another embodiment;

FIG. 9 is a side view illustrating an example of a main part of aprojection optical system provided in an image display apparatusaccording to still another embodiment;

FIG. 10 is a ray diagram illustrating features of light projected by theprojection optical system;

FIG. 11 is a plan view illustrating an example of a reflection imagedisplay device provided in the image display apparatus according to theembodiment;

FIG. 12 is a side view illustrating an example of a lens optical systemprovided in the image display apparatus according to the embodiment;

FIG. 13 is a side view illustrating an example of a main part of anprojection optical system provided in an image display apparatusaccording to an embodiment;

FIG. 14 is a ray diagram illustrating features of light projected by theprojection optical system;

FIG. 15 is a side view illustrating another example of a main part of anprojection optical system provided in an image display apparatusaccording to another embodiment;

FIG. 16 is a side view illustrating another example of a main part of anprojection optical system provided in an image display apparatusaccording to another embodiment;

FIG. 17 is a ray diagram illustrating the trajectory of light projectedby the projection optical system;

FIG. 18 is a plan view illustrating an example of a reflection imagedisplay device provided in the image display apparatus according to theembodiment;

FIG. 19 is a side view illustrating an example of a lens optical systemprovided in the image display apparatus according to the embodiment;

FIG. 20 is a table illustrating a configuration of a coaxial projectionoptical system;

FIG. 21 is a table illustrating aspherical coefficients of respectiveplanes;

FIGS. 22A and 22B represent a table illustrating coefficients forforming a reflection plane of the second mirror 10; and

FIG. 23 is a table illustrating a layout of a first mirror 9, a secondmirror 10 and a dustproof glass 11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments are described below with reference to theaccompanying drawings.

Preferred embodiments of an image display apparatus are described belowwith reference to the accompanying drawings. FIG. 1 is a side viewillustrating an example of a main part of an optical engine provided inan image forming apparatus according to an embodiment. In the followingdescription, a Z-axis represents an optical axis direction of aprojection optical system, a Y-axis represents an optical axis directionof an illumination optical system, and an X-axis represents an opticalaxis direction orthogonal to the Z-axis and the Y-axis.

In FIG. 1, a projector 100 includes an illumination optical systemconfigured to illuminate a digital micromirror device (DMD) 7 serving asa reflection image display device utilizing light emitted from a lamp 1serving as a light source, a projection optical system configured toproject light reflected off the DMD in a direction toward a screen 20serving as a projection plane. FIG. 1 only illustrates a lens opticalsystem 8 that is a part of the projection optical system.

Note that the DMD serving as a reflection image display device isemployed as an example of the image display device in the followingembodiments. However, the image display apparatus according to theembodiments includes an image display device that is not limited to theDMD. The image display apparatus according to the embodiments mayinclude other types of image display devices such as a liquid crystalpanel.

Next, an illumination optical system provided in the projector 100 isdescribed below. Light emitted from the lamp 1 serving as a light sourceis converged or collected by a reflector 2 at an entrance of anintegrator rod 3. The integrator rod 3 may, for example, be a light pipeformed by combining four mirrors in a form of a tunnel. The lightconverged at the entrance of the integrator rod 3 repeatedly reflectsoff surfaces of the mirrors inside the integrator rod 3, and hence,light intensity may become uniform at an exit of the integrator rod 3.

The exit of the integrator rod 3 may serve as a surface light sourceconfigured to emit light having uniform light intensity. Accordingly, alight source image from the surface light source may be formed in aneffective image region of the DMD 7 serving as the image display devicevia a DMD illumination lens 4, a first reflector 5, and a secondreflector 6. The DMD illumination lens 4 serves as an optical device foreffectively applying light to the effective image region of the DMD 7.The first reflector 5 is formed of a plane mirror and the secondreflector 6 is formed of a curved mirror (i.e., a concave mirror).

The light emitted from the integrator rod 3 passes through the front ofthe DMD illumination lens 4 and reflects off the first reflector 5 sothat the light reflected off the first reflector 5 is reflected in alower right direction to travel in a direction toward the secondreflector 6 as illustrated in FIG. 1. The light reflected off the secondreflector 6 illuminates a surface of the DMD 7 and the lightilluminating the surface of the DMD 7 further reflects off a mirrorwithin the effective image region of the DMD 7. Hence, the reflectedluminous flux corresponding to image projection light passes rightbeside the second reflector 6 to be incident upon a lens optical system8 that constitutes a projection optical system. The illumination opticalsystem includes the lamp 1 and the second reflector 6.

Since the illumination optical system illuminates the DMD 7 with auniform amount of luminous light, the DMD 7 exhibits uniformillumination distribution. Hence, a projection image enlarged by thelight projected from the illumination optical system via the DMD 7 alsoexhibits uniform illumination distribution.

The DMD 7 is composed of numerous micromirrors, respective angles ofwhich may vary within a range of +12 to −12 degrees. For example, if theangle of each micromirror is −12 degrees, light reflected off themicromirror may be configured to be incident on the projector lens. Thisstate is called an “ON state”. For example, if the angle of themicromirror is +12 degrees, light reflected off the micromirror may beconfigured not to be incident on the projector lens. This state iscalled an “OFF state”.

The micromirrors of the DMD 7 correspond to pixels of an image displayedon a projection plane. Accordingly, projection light (projection imagelight) forming an image displayed on the screen 20 may be projected viathe projection optical system by controlling angles of inclination ofthe micromirrors of the DMD 7.

FIG. 1 only illustrates a lens optical system 8, and illustration of amirror optical system included in the projection optical system isomitted from FIG. 1. The lens optical system 8 includes a projector lenscomposed of plural lenses and a lens barrel holding the projector lens.Note that illustration of the lens barrel is omitted from FIG. 1. Notealso that the not-illustrated mirror optical system includes a mirrorconfigured to reflect off the projection luminous flux obtained from theprojector lens in a direction toward the screen 20.

FIRST EMBODIMENT

Next, a projection optical system provided in an image display apparatusaccording to a first embodiment is described. FIG. 2 is a schematicdiagram illustrating an enlarged main part of the projection opticalsystem provided in an image display apparatus according to the firstembodiment. Note that illustration of an illumination optical system isomitted from FIG. 2. FIG. 2 illustrates the projection optical systemconfigured to project an entire effective image region onto a screen 20(not illustrated) serving as a projection surface while all themicromirrors of the DMD 7 are in ON states. In FIG. 2, a projectionluminous flux 14 is incident on the lens optical system 8 from an end ofthe effective image region of the DMD 7. The projection luminous flux isexpressed as two lines reaching the screen 20 (not illustrated) via thefirst mirror 9 and the second mirror 10 that constitutes parts of amirror optical system 8.

The lens optical system 8 is composed of plural lenses assembled insidea lens barrel 81. The projection luminous flux 14 is converged insidethe lens barrel 81, and the converged projection luminous flux 14 thentravels in a direction toward the first mirror 9 while diffusing. Thefirst mirror 9 is not limited to a plane mirror. If the first mirror 9is a convex mirror, the projection luminous flux 14 reflected off thefirst mirror 9 may exhibit increased divergence. In this case, theprojection luminous flux 14 reflected off the first mirror 9 may, forexample, collide with the lens barrel 81 in the middle of the routedirected toward the second mirror 10. As a result, the projectionluminous flux 14 may be blocked off (be shielded). Accordingly, it ispreferable that the first mirror 9 be a plane mirror or a concavemirror.

Further, the second mirror 10 configured to reflect off the projectionluminous flux 14 obtained from the first mirror 9 in a direction towardthe screen 20 may need to be a concave mirror. If the second mirror 10is a plane mirror or a convex mirror, the reflected projection luminousflux 14 may be diffused. Hence, the reflected projection luminous flux14 does not travel in a direction toward the screen 20(not-illustrated).

That is, if the first mirror 9 is a plane mirror or a convex mirror, andthe second mirror 10 is a concave mirror, the projection luminous flux14 reflected off the second mirror 10 diffuses after being converged orcollected between the screen 20 and the second mirror 10. Accordingly,the projection luminous flux 14 is capable of being projected onto thescreen 20.

In order to display an enlarged image on the screen 20 while placing theprojector 100 extremely close to the screen 20, it may be necessary toenhance the divergence of the luminous flux incident on the mirroroptical system from the lens optical system 8, converge the projectionluminous flux 14 after being reflected off the second mirror 10 (concavemirror) provided in the mirror optical system, and set a convergingposition 15 at a position close to the second mirror 10 and distant fromthe screen 20.

If the converging position 15 is distant from the screen 20 but is notclose to the second mirror 10, divergence of the projection luminousflux 14 obtained from the projector 100 set at a position extremelyclose to the screen 20 will not be sufficient. Accordingly, a largeimage will not be displayed on the screen 20.

For example, as illustrated in FIG. 3, if the converging position 15 isdistant from the screen 20 and is close to the second mirror 10, theprojection luminous flux 14 from the projector 100 is sufficientlydivergent even if the projector 100 is set at a position extremely closeto the screen 20. Accordingly, the sufficiently divergent projectionluminous flux 14 may be displayed as an enlarged image on an entiresurface of the screen 20.

The converging position 15 is described further in detail below. It ispreferable that the converging position 15 be farther distant from thescreen 20 (i.e., closer to the second mirror 10); however, it isundesirable that the converging position 15 be too close to the secondmirror 10. It is preferable that the converging position 15 be locatedat a position closer to the first mirror 9 than the second mirror 10serving as a concave mirror.

The reason why the converging position 15 is desirable to be located ata position closer to the first mirror 9 than the second mirror 10 isgiven below with reference to FIG. 4. FIG. 4 is an enlarged viewillustrating a main part of the projection optical system provided inthe projector 100 according to the embodiment. In this example, theconverging position 15 is located at a position closer to the secondmirror 10 than the first mirror 9. The projection luminous flux 14 isconverged at the converging position 15, and the converged projectionluminous flux 14 is then diffused at once. In this case, if theconverging position 15 is not close to the first mirror 9 but close tothe second mirror 10 as illustrated in FIG. 4, the first mirror 9 may beplaced on an optical path of the projection luminous flux 14 thatdiffuses from the converging position 15. In this case, part of theprojection luminous flux 14 may be blocked off by the first mirror 9.

In FIG. 5, in comparing projection light 141 that strikes an edge ofprojection luminous flux 14 reflected off the second mirror 10 andtraveling in a direction toward the screen 20 and projection light 142that strikes another edge of the projection luminous flux 14, theprojection light 141 is reflected off the second mirror 10 and projectedonto the screen 20 via the converging position 15, whereas theprojection light 142 reflected off the second mirror 10 travels to theconverging position 15 but is then blocked off by the first mirror 9. Asa result, an image expected to be displayed on the screen 20 with theprojection luminous flux 14 may have missing parts. With this reason, itis preferable that the converging position 15 be close to the firstmirror 9.

If the converging position 15 is not close to the first mirror 9 but isaway from the first mirror 9, and is close to the second mirror 10 asdescribed above, a part of the projection luminous flux 14 may beblocked off by the projection optical system. As a result, an enlargedimage will not be able to be displayed on the screen 20. In theprojector 100 according to the embodiment, the converging position 15 isset at a position farther away from the screen 20 and closer to thesecond mirror 10, and is closer to the first mirror 9. However, it isundesirable that the converging position 15 be much too close to thesecond mirror 9. With this configuration, the projector 100 according tothe embodiment may be capable of displaying an enlarged image on thescreen 20 at an extremely close range of the screen 20.

Further, if the converging position 15 is not close to the first mirror9 but close to the second mirror 10, the projection light 142 that is apart of the projection luminous flux 14 has an angle to bow relative tothe screen 20 that is parallel to an X-Z plane, compared to theprojection light 141 reaching the screen 20. In other words, when anangle between a first edge (projection light 141) of the projectionluminous flux 14 reflected off the second mirror 10 and a normal line(i.e., a line normal to tangential line) of the screen 20 and an anglebetween a second edge (projection light 142) and the normal line of thescreen 20 have different signs (for the angles), the projection luminousflux 14 is blocked off by the lens optical system 8.

Accordingly, in the projector 100 according to the embodiment, each ofangles formed between rays of light incident on an image projected onthe screen serving as the projection surface at the center in ahorizontal direction associated with the projection luminous flux 14 andthe normal of the screen 20 has the same sign.

SECOND EMBODIMENT

Next, a projection optical system provided in an image display apparatusaccording to a second embodiment is described. FIG. 6 is a schematicdiagram illustrating an enlarged main part of the projection opticalsystem provided in an image display apparatus according to secondembodiment.

The purposes of arranging dustproof glass 11 above the second mirror 10in the projection optical system provided in the image display apparatusaccording to the second embodiment is to protect the second mirror 10 ofthe concave mirror and to prevent particles such as dust from intrudinginto the lens optical system 8. Note that the quality of the projectedimage will not deteriorate by arranging the dustproof glass 11 in theprojection optical system.

It is preferable that the dustproof glass 11 be as small as possible inthe projection optical system provided in the image display apparatusaccording to the second embodiment. That is, if the dustproof glass 11is large, the projection optical system itself is large, which mayresult in an increase in the size of the projector 100.

The projection luminous flux 14 reflected off the second mirror 10 totravel toward the screen 20 (not illustrated) is converged once, and theonce converged projection luminous flux 14 is then diffused. In order toachieve the purpose of arranging the dustproof glass 11 in theprojection optical system without increasing the size of the dustproofglass 11, the projection optical system may be configured such that theconverging position 15 is located close to the dustproof glass 11. Asillustrated in FIG. 6, if the converging position 15 of the projectionluminous flux 14 is located close to the dustproof glass 11, the size ofthe dustproof glass 11 is not necessarily be increased. Hence, theprojection optical system may be reduced in size. However, if theconverging position 15 of the projection luminous flux 14 is locatedaway from the dustproof glass 11, the projection luminous flux 14diffuses at once. Hence, the size of the dustproof glass 11 may need tobe increased.

That is, in the projector 100 according to the embodiment, it ispreferable to locate the converging position 15 at a position fartheraway from the screen 20 and close to the second mirror 10, and at aposition closer to the first mirror 9 than the second mirror and closerto the dustproof glass 11 in order to satisfy the following threerequirements: (1) the projection luminous flux 14 traveling in adirection from the second mirror 10 toward the screen 20 will not beblocked off, (2) the size of the entire projection optical systemincluding the lens optical system 8, the first mirror 9, and the secondmirror 10 may be reduced, and (3) the size of the dustproof glass 11 maybe reduced. Note that the converging position 15 is a position at whichthe projection luminous flux 14 is converged after the projectionluminous flux 14 has reflected off the second mirror 10.

THIRD EMBODIMENT

Next, a projection optical system provided in an image display apparatusaccording to a third embodiment is described. FIG. 7 is a schematicdiagram illustrating features of the third embodiment, which is anenlarged main part of the projection optical system provided in theimage display apparatus according to the third embodiment. The preferredlocation of the converging position 15 being closer to the dustproofglass 11 has been already described in the aforementioned secondembodiment. If the first mirror 9 in the second embodiment illustratedin FIG. 6 rotates in a counterclockwise direction on paper, theconverging position 15 is located at a position even closer to thedustproof glass 11 and the first mirror 9 as indicated by a broken linein FIG. 7. However, with this configuration, the projection luminousflux 14 that has reflected off the first mirror 9 may be susceptible tobeing blocked off by the lens optical system 8 or the lens barrel 81.

Specifically, if the lens arranged closest to the first mirror 9 amongthe lenses of the lens optical system 8 is concave (i.e., a concavelens), the projection luminous flux 14 reflected off the first mirror 9is highly likely to strike an outer circumferential part of the concavelens.

On the other hand, if the angle of the first mirror 9 is rotated in aclockwise direction on paper, the projection luminous flux 14 may beadjusted such that the projection luminous flux 14 that has reflectedoff the first mirror 9 is not blocked off by the lens optical system 8or the lens barrel 8. However, if the angle of the first mirror 9 isrotated in a clockwise direction on paper, an optical path of theprojection luminous flux 14 that has reflected off the first mirror 9 isshifted in a direction away from an X-Y plane. In this case, the secondmirror 10 may need to be shifted according to the shifted optical pathof the projection luminous flux 14. That is, the second mirror 10 mayneed to be shifted in an upward direction on paper. If the second mirror10 is shifted in the upward direction on paper, the projection opticalsystem may become bulky.

Accordingly, in order to reduce the size of the projection opticalsystem, the projection optical system may preferably be configured suchthat the projection luminous flux 14 that has reflected off the firstmirror 9 passes through a position closest possible to the lensconstituting the lens optical system 8 or the lens barrel 81 to reachthe second mirror 10 while locating the converging position 15 close tothe dustproof glass 11.

In order to obtain the projection optical system having the aboveconfiguration, a configuration of the lens optical system 8 may need tobe improved. FIG. 8 is a view illustrating an example in which a lenslocated closest to the first mirror 9 among projector lenses of the lensoptical system 8 has a convex lens surface. As illustrated in FIG. 8, ina case of the lens 80 having a convex lens surface, even if theprojection luminous flux 14 passes through a position close to the lens80, the projection luminous flux 14 will not be blocked off by the outercircumferential part of the lens 80 or an upper end of the lens barrel81. Further, even if the projection luminous flux 14 that has reflectedoff the first mirror 9 has high divergence, the projection luminous flux14 may be able to reach the second mirror 10.

According to the aforementioned third embodiment, aberration correctionmanaging the projection luminous flux 14 having increased divergence maybe performed, and error sensitivity may be reduced.

FOURTH EMBODIMENT

Next, a projection optical system provided in an image display apparatusaccording to a fourth embodiment is described. FIG. 9 is a schematicdiagram illustrating features of the fourth embodiment, which is anenlarged main part of the projection optical system provided in theimage display apparatus according to the fourth embodiment that managesdivergence of the reflected luminous flux much higher than thedivergence managed by the projection optical system provided in theimage display apparatus according to the third embodiment.

In FIG. 9, a lens optical system 8 a includes an atypical lens 80 aarranged closest to the first mirror 9 and a lens barrel 81 a holdingthe atypical lens 80 a and the like. As illustrated in FIG. 9, among thelenses constituting the lens optical system 8 a, the atypical lens 80 aarranged closest to the first mirror 9 is cut out such that a diameterof the atypical lens 80 a is reduced by half of the diameters of otherlenses. As a result, a cutout part is formed in an end part on the firstmirror 9 side of the lens optical system 80 a. With this configuration,even if the projection luminous flux 14 that has reflected off the firstmirror 9 has high divergence and thus exhibits a diffused optical path,the projection luminous flux 14 will not be blocked off by the lensoptical system 8 a and may be able to reach the second mirror 10.

Accordingly, a route allowing the projection luminous flux 14 to passthrough may be widened by forming the cutout part in an end part of thelens barrel 81 a and the lens 80 a closest to the first mirror 9. Hence,the projector 100 may have the projection optical system that is reducedin size and weight, and may be capable of projecting an enlarged imageonto the screen 20 at an extremely close range of the screen 20.

As is clear from rays of light illustrated in FIG. 9, the usable rangeof the lens 80 a is a remaining part opposite to the cutout part of thelens 80 a. Accordingly, the definition and the brightness of the imageprojected on the screen 20 will not be affected.

FIFTH EMBODIMENT

Next, a projection optical system provided in an image display apparatusaccording to a fifth embodiment is described. FIG. 10 is a ray diagramillustrating trajectory of light projected by the projection opticalsystem. In FIG. 10, an optimal converging position 15 is formed byarranging the lens optical system 8, the first mirror 9, the secondmirror 10 and the dustproof glass 11, and seven rays of light are tracedfrom each of fifteen points on the DMD 7 illustrated in FIG. 11. FIG. 11is a plan view of the DMD 7. In FIG. 11, among plural points on a planeof the DMD 7, a point 71 placed at a midpoint in an X-axis direction andat a lower end of the plane of the DMD 7 in a Y-axis direction iseccentrically arranged in the Y-axis direction. The amount ofeccentricity is 1.56 mm.

The illustration of the embodiment continues by referring back to FIG.10. As illustrated in FIG. 10, the collecting power of light at theconverging position 15 is not narrowed as a spot; however, theprojection optical system may be reduced in size by arranging thedustproof glass 11 at a position near the converging position 15.

Further, the projection luminous flux 14 will not be blocked off by thefirst mirror 9 by locating the converging position 15 at a positionclose to the first mirror 9, that is, by locating the convergingposition 15 close to an infinitely-wide virtual plane including areflection plane of the first mirror 9. In addition, since the firstmirror 9 may be arranged without having a distance from the secondmirror 10, the projection optical system may be reduced in size.

Note that a projector having a configuration differing from that of theprojector 100 utilized in the above embodiment is considered. Theprojector is configured such that the projection luminous flux 14emitted from the DMD 7 is not reflected off the first mirror 9 but theprojection luminous flux 14 having passed through the lens opticalsystem 8 strikes the second mirror 10. In the projector having such aconfiguration, if the projection optical system is arranged on anexterior of the projector, a main body of the projector may collide withthe screen 20. Accordingly, the projector having this configuration maynot be placed at a close range of the screen 20. That is, the projector100 utilized in the image display apparatus according to the aboveembodiment is configured such that the projection luminous flux 14exhibiting high divergence is reflected via the first mirror 9 and thesecond mirror 10, and projected onto the screen 20. Accordingly, theimage display apparatus according to the above embodiment having theprojector 100 may be capable of projecting an image on the screen 20from a position extremely close to the screen 20.

FIG. 12 illustrates a configuration example of the lens optical system8. In FIG. 12, the optical axis of the lenses is determined as a Z-axisand two other axis orthogonal to the Z-axis are determined as an X-axisand a Y-axis, respectively. The lens optical system 8 illustrated inFIG. 12 is a coaxial optical system in which the respective optical axesof the lenses are in the same straight lines.

In FIG. 18, among plural points on a plane of the DMD 7 illustrated inFIG. 18, a point 71 on a lower end of the plane of the DMD 7 in theY-axis direction is eccentrically arranged in the Y-axis direction, andthe amount of eccentricity is 1.56 mm. That is, in FIG. 12, the opticalaxes of the DMD 7 are located 1.56 mm below the lower end of the DMD 7.

SIXTH EMBODIMENT

Next, a projection optical system provided in an image display apparatusaccording to a sixth embodiment is described. FIG. 13 is a schematicdiagram illustrating an enlarged main part of the projection opticalsystem provided in the image display apparatus according to the sixthembodiment. Note that illustration of an illumination optical system isomitted from FIG. 13. FIG. 13 illustrates the projection optical systemconfigured to project an entire effective image region onto a screen 20(not illustrated) serving as a projection surface while all themicromirrors of the DMD 7 are in ON states. In FIG. 13, a projectionluminous flux 14 is incident on the lens optical system 8 from an end ofthe effective image region of the DMD 7. The projection luminous flux isexpressed as two lines reaching the screen 20 (not illustrated) via thefirst mirror 9 and the second mirror 10 that constitutes a mirroroptical system 8.

The lens optical system 8 is composed of plural lenses assembled insidea lens barrel 81. The projection luminous flux 14 is converged insidethe lens barrel 81 and the converged projection luminous flux 14 thentravels in a direction toward the first mirror 9 while diffusing. Thefirst mirror 9 is not limited to a plane mirror. If the first mirror 9is a convex mirror, the projection luminous flux 14 that has reflectedoff the first mirror 9 may exhibit increased divergence. In this case,the projection luminous flux 14 reflected off the first mirror 9 may,for example, collide with the lens barrel 81 in the middle of the routedirected toward the second mirror 10. As a result, the projectionluminous flux 14 may be blocked off (be shielded). Accordingly, it ispreferable that the first mirror 9 be a plane mirror or a concavemirror.

Further, the second mirror 10 configured to reflect off the projectionluminous flux 14 having reflected off the first mirror 9 in a directiontoward the screen 20 may need to be a concave mirror. If the secondmirror 10 is a plane mirror or a convex mirror, the reflected projectionluminous flux 14 may be diffused. In this case, the reflected projectionluminous flux 14 will not travel in a direction toward the screen 20.

That is, if the first mirror 9 is a plane mirror or a convex mirror, andthe second mirror 10 is a concave mirror, the projection luminous flux14 that has reflected off the second mirror 10 diffuses after beingconverged or collected between the screen 20 and the second mirror 10.Accordingly, the projection luminous flux 14 is capable of beingprojected onto the screen 20.

In order to display an enlarged image on the screen 20 while placing theprojector 100 extremely close to the screen 20, it may be necessary toenhance the divergence of the luminous flux incident on the mirroroptical system from the lens optical system 8, converge the projectionluminous flux 14 after being reflected off the second mirror 10 (concavemirror) provided in the mirror optical system and set a convergingposition 15 to a position close to the second mirror 10 and distant fromthe screen 20.

If the converging position 15 is distant from the screen 20 but is notclose to the second mirror 10, divergence of the projection luminousflux 14 obtained from the projector 100 set at a position extremelyclose to the screen 20 will not be sufficient. Accordingly, a largeimage will not be displayed on the screen 20.

For example, as illustrated in FIG. 14, if the converging position 15 isdistant from the screen 20 and is close to the second mirror 10, theprojection luminous flux 14 from the projector 100 is sufficientlydivergent even if the projector 100 is set at a position extremely closeto the screen 20. Accordingly, the sufficiently divergent projectionluminous flux 14 may be displayed as an enlarged image on an entiresurface of the screen 20.

The illustration of the embodiment continues by referring back to FIG.13. As illustrated in FIG. 13, since the lens optical system 8 isvertically arranged along a direction of gravitational force, the lensclosest to the first mirror 9 may preferably have a convex surface. Withthe convex surface of the lens, dust will not be easily attached to thelens surface and dust attached to the lens surface may spontaneouslyfall along the lens surface by gravitational force in a downwarddirection. Further, dust will not intrude inside the lens barrel 81.Hence, among the lenses of the lens optical system 8, a lens surface ofa lens closest to the DMD 7 may rarely acquire dust.

In the projector 100 according to the embodiment described above, theconverging position 15 is set at a position farther away from the screen20 and closer to the second mirror 10. Accordingly, even if theprojector 100 is placed at a position extremely close to the screen 20,an enlarged image may be displayed on the screen 20. Further, even ifthe lens optical system 8 is vertically arranged in the projector 100,dust will not be reflected in a projected image.

SEVENTH EMBODIMENT

Next, a projection optical system provided in an image display apparatusaccording to a seventh embodiment is described. In the projectionoptical system illustrated in FIG. 13, if an area of the first mirror 9is denoted by S1, an area of the second mirror 10 is denoted by S2, S1and S2 has a relationship represented by S1<S2, which may be representedby a configuration illustrated in FIG. 13. The smaller the proportion ofthe area of the mirror and the particle size of dust, the lower theadverse effect on the reflection of dust on the screen (e.g., denoted byan “area H”) or the amount of light on the screen may be. For example,the adverse effect on the screen may be greater when dust having aparticle size of 0.01 mm is attached to the first mirror 9 than when thedust having the same particle size attached to the second mirror 10.

Hence, in order to reduce the adverse effect on the screen such as thedust reflecting on the screen or the reduction in the amount of light onthe screen due to the dust reflection without changing the area H of thescreen, it is preferable to increase the area S1 of the first mirror 9.However, it may be necessary to increase the angle of view of the lensoptical system 8 to increase the area S1 of the first mirror 9. In orderto downsize the entire configuration of the projection optical systemafter having increased the angle of view of the lens optical system 8,it is preferable that a concave lens group be applied to a lens grouplocated close to the first mirror 9 and a convex lens group havingpositive refractive power be applied to a lens group located close tothe DMD 7 among lens groups of the lens optical system 8.

In this case, the divergence of the projection luminous flux 14 isfurther be increased, and hence, it may be possible that the projectionluminous flux 14 that has reflected off the first mirror 9 may strikethe lens optical system 8 to be blocked off by the lens optical system8. Hence, in order to overcome such an outcome, a possible obstacle partof the lens optical system 8 may be eliminated from the optical path ofthe projection luminous flux 14 reflected off the first mirror 9 andtraveling toward the second mirror 10, such that the projection luminousflux 14 will not be blocked off by the lens optical system 8. Thus, anatypical lens may be applied to a lens closest to the first mirror 9among the lenses constituting the lens optical system 8 such that thelens optical system 8 will not block off the projection luminous flux14.

FIG. 15 is a side view illustrating an example of a projection opticalsystem provided in an image display apparatus according to theembodiment in which an atypical lens is applied to a lens closest to thefirst mirror 9. In FIG. 15, the lens optical system 8 a includes theatypical lens 80 a arranged closest to the first mirror 9 and the lensbarrel 81 a holding the atypical lens 80 a and the like. As illustratedin FIG. 15, among the lenses constituting the lens optical system 8 a,approximately a half of the atypical lens 80 a arranged closest to thesecond mirror 9 is cut out and the other half remain unremoved. Withthis configuration, any part of the lens optical system 8 a will notinterfere with the optical path of the projection luminous flux 14reflected off the first mirror 9 and traveling toward the second mirror14. In addition, even if the projection luminous flux 14 reflected offthe first mirror 9 has higher diffusability, the projection luminousflux 14 will not be blocked off by the lens optical system 8 a and maybe able to reach the second mirror 10.

Accordingly, the optical path allowing the projection luminous flux 14to pass through may be widened by forming a cutout part in an end partof the lens barrel 81 a and the lens 80 a closest to the first mirror 9.Hence, the projector 100 may have the projection optical system that isreduced in size and weight and may be capable of projecting an enlargedimage on the screen 20 at an extremely close range of the screen 20.

As is clear from rays of light illustrated in FIG. 15, the usable rangeof the lens 80 a is a remaining part opposite to the cutout part of thelens 80 a. Accordingly, the definition and the brightness of the imageprojected on the screen 20 will not be affected.

Note that the lens 80 a may be formed of a plastic lens or a mold lensthat originally includes a cutout part.

EIGHTH EMBODIMENT

Next, a projection optical system provided in an image display apparatusaccording to an eighth embodiment is described. FIG. 16 is a schematicdiagram illustrating an enlarged main part of the projection opticalsystem provided in the image display apparatus according to the eighthembodiment.

The purposes of arranging dustproof glass 11 above the second mirror 10in the projection optical system provided in the image display apparatusaccording to the eighth embodiment is to protect the second mirror 10 ofthe concave mirror and to prevent particles such as dust from intrudinginto the lens optical system 8. Note that the quality of the projectedimage will not deteriorate by arranging the dustproof glass 11 in theprojection optical system.

It is preferable that the dustproof glass 11 be as small as possible inthe projection optical system provided in an image display apparatusaccording to the eighth embodiment. That is, if the dustproof glass 11is large, the projection optical system itself is large, which mayincrease the size of the projector 100.

The projection luminous flux 14 reflected off the second mirror 10 totravel toward the screen 20 (not illustrated) is converged once, and theonce converged projection luminous flux 14 is then diffused. In order toachieve the purpose of arranging the dustproof glass 11 in theprojection optical system without increasing the size of the dustproofglass 11, the projection optical system may be configured such that theconverging position 15 is located close to the dustproof glass 11. Asillustrated in FIG. 16, if the converging position 15 of the projectionluminous flux 14 is located close to the dustproof glass 11, the size ofthe dustproof glass 11 may not necessarily be increased. Hence, theprojection optical system may be reduced in size. However, if theconverging position 15 of the projection luminous flux 14 is locatedaway from the dustproof glass 11, the projection luminous flux 14diffuses at once. Hence, the size of the dustproof glass 11 may need tobe increased.

That is, in the projector 100 according to the embodiment, it ispreferable to locate the converging position 15 at a position close tothe dustproof glass 11 and the first mirror 9 of the plane mirror inorder to satisfy the following three requirements: (1) the projectionluminous flux 14 traveling in a direction from the second mirror 10 tothe screen 20 will not be blocked off, (2) the size of the entireprojection optical system including the lens optical system 8, the firstmirror 9 and the second mirror 10 may be reduced, and (3) the size ofthe dustproof glass 11 may be reduced.

Further, when dust is accumulated in the converging position 15, theamount of light in the screen 20 may be decreased. Accordingly, thedustproof glass 11 may be arranged such that the dustproof glass 11covers the converging position 15 while the dustproof glass 11 isinclined to the entire projection luminous flux 14. For example, asillustrated in FIG. 16, the dustproof glass 11 may be arranged parallelto a plane of the DMD 7.

NINTH EMBODIMENT

Next, a projection optical system provided in an image display apparatusaccording to a ninth embodiment is described. FIG. 17 is a ray diagramillustrating trajectory of light projected by the projection opticalsystem. In FIG. 17, seven rays of light are traced from each of fifteenpoints on the DMD 7 illustrated in FIG. 18. In FIG. 17, referencenumerals 16 and 17 each denote an optimal position at which anintermediate image is formed in the optical path of the projector 100 inthis embodiment.

FIG. 18 is a plan view of the DMD 7. In FIG. 18, among plural points ona plane of the DMD 7, a point 71 placed at a midpoint in an X-axisdirection and at a lower end of the plane of the DMD 7 in a Y-axisdirection is eccentrically arranged in the Y-axis direction. The amountof eccentricity is 1.56 mm.

The illustration of the embodiment continues by referring back to FIG.17. FIG. 17 is a ray diagram illustrating the trajectory of lightprojected on the screen by the projection optical system utilized in theimage display apparatus according to the ninth embodiment. Asillustrated in FIG. 17, the projection optical system includes twocoupling optical systems including the lens optical system 8 and thesecond mirror 10 formed of a concave mirror. The lens optical system 8includes a function to form an image (intermediate image) of the DMD 7.The second mirror 10 serves as a function to form an intermediate imageof the DMD 7 on the screen 20.

The projection optical system having the two coupling optical systemsexhibits a function to form a “definite” or “sharp” image of the DMD 7on the screen 20. In this case, the intermediate image of the DMD 7 maybe a “blurred” or “fuzzy” image. It may be rather preferable that theintermediate image be a “blurred” image in order to prevent particles ofdust from being reflected on the screen 20. That is, if the intermediateimage is “definite” or “sharp”, the “definite intermediate image” isreflected off the second mirror 10 to form a “definite image” on thescreen 20.

If dust is present around the first mirror 9 of a plane mirror on whichthe intermediate image is formed, an image of the dust is definitelyreflected on the screen 20. If, on the other hand, the intermediateimage is a “blurred” image, the blurred image reflects off the secondmirror 10 to definitely form a “blurred image” on the screen 20.Accordingly, even if dust is attached to the first mirror 9, the imageof the dust formed on the screen 20 is the indistinctive “blurred image”of the dust. Hence, it is preferable to form the “blurred image” in thelens optical system 8 so as to definitely form the “blurred intermediateimage” reflected off the mirror 10 on the screen 20.

However, it may be difficult to form a “blurred” intermediate imagecomposed of pixels located close to the optical axis. Accordingly, theprojection optical system utilized in the image display apparatusaccording to the ninth embodiment is configured such that theintermediate image composed of pixels located close to the optical axisis not formed on the first mirror of the plane mirror.

In FIG. 17, the reference numeral 16 indicates a position at which anintermediate image composed of pixels located close to the optical axis.Likewise, the reference numeral 17 indicates a position at which anintermediate image composed of pixels located distant (away) from theoptical axis. In the DMD 7, an intermediate image composed of pixelslocated farthest from the optical axis of the lens optical system 8 mayspontaneously have increased aberration. Accordingly, since an opticalspot diameter becomes larger, dust reflected on the mirror surface maybe smaller.

The second mirror 10 of the concave mirror is configured to form anintermediate image formed in the lens optical system 8 on the screen 20.Hence, if dust is present around the position 16, an image of the dustmay be formed on the screen 20. Specifically, if dust is present at aposition of the definite intermediate image having a small spotdiameter, an image of the dust may be definitely reflected on the screen20. If, on the other hand, dust is present at a position of the definiteintermediate image having a large spot diameter, an image of the dustmay be reflected as a “blurred” image of the dust so as not to bedistinctively reflected on the screen 20. Accordingly, it is preferableto form an intermediate image composed of pixels located away from theoptical axis between the lens optical system 8 and the first mirror 9.With such a configuration, it may be possible to obtain an image displayapparatus capable of projecting an enlarged image onto the screen 20 byacquiring large curvature of field and minimizing a distance between thescreen 20 and the projector 100.

If the intermediate image composed of pixels located close to theoptical axis to distant from the optical axis is formed on a reflectionplane of the first mirror 9, an intermediate image is formed such thatthe intermediate image has a large spot diameter on a reflection surfaceof the second mirror 10 to blur the intermediate image, and that theintermediate image is formed between the first mirror 9 and the secondmirror 10. With this configuration, dust attached to the first mirror 9will not be reflected on the screen 20.

A spot diameter may be adjusted as an optimal size by applying twoaspheric lenses to lenses of the lens optical system 8 on the firstmirror 9 side. Further, aspheric lenses may be applied to the lenseslocated near a diaphragm so as to increase a spot diameter of theintermediate image composed of pixels located close to the optical axiswhile decreasing a spot diameter of an image composed of pixels locatedclose to the optical axis on the projection surface of the screen 20.

Note that a projector having a configuration differing from that of theprojector 100 utilized in the above embodiment is considered. Theprojector is configured such that the projection luminous flux 14emitted from the DMD 7 is not reflected off the first mirror 9 but theprojection luminous flux 14 having passed through the lens opticalsystem 8 strikes the second mirror 10. In the projector having such aconfiguration, if the projection optical system is arranged on anexterior of the projector, a main body of the projector may collide withthe screen 20. Accordingly, the projector having this configuration maynot be placed at a close range of the screen 20. By contrast, theprojector 100 utilized in the image display apparatus according to theabove embodiment is configured such that the projection luminous flux 14exhibiting high divergence is reflected via the first mirror 9 and thesecond mirror 10 and projected onto the screen 20. Accordingly, theimage display apparatus according to the above embodiment having theprojector 100 may be capable of projecting an image on the screen 20from a position extremely close to the screen 20.

FIG. 19 illustrates a configuration example of the lens optical system8. In FIG. 19, the optical axis of the lenses is determined as a Z-axisand two other axis orthogonal to the Z-axis are determined as an X-axisand a Y-axis, respectively. The lens optical system 8 illustrated inFIG. 19 is a coaxial optical system in which the respective optical axesof the lenses are in the same straight lines.

In FIG. 18, among plural points on a plane of the DMD 7 illustrated inFIG. 18, a point 71 on a lower end of the plane of the DMD 7 in theY-axis direction is eccentrically arranged in the Y-axis direction, andthe amount of eccentricity is 1.56 mm. That is, in FIG. 19, the opticalaxes of the DMD 7 are located 1.56 mm below the lower end of the DMD 7.

As illustrated in FIG. 19, a lens group located closest to the DMD 11serving as an image display device among plural lens groups constitutingthe lens optical system 8 includes a diaphragm 82. The amount ofluminous flux emitted from the DMD11 and reaching the not-illustratedscreen 20 may be determined based on the diaphragm 82. Further, theangle of field may be increased by arranging the diaphragm 82 in thelens group located closest to the DMD11. With such a configuration, theeffect of the dust or particles reflecting on the screen may be reducedowing to an increase of the diffusion of the luminous flux at a positionof the lens closest to a mirror optical system where dust or particlesare most likely to accumulate or an increase of the diffusion of theluminous flux on the reflection plane.

Next, specific numerical examples of the projection optical system areillustrated. FIG. 20 is a table illustrating a configuration of theaforementioned coaxial optical system.

In the table of FIG. 20, planes 4, 5, 21, 22, 23 and 24 are aspherical,and FIG. 21 illustrates aspherical coefficients of the respectiveplanes.

The aspherical planes are computed by the application of the asphericalcoefficients illustrated in the table of FIG. 21 based on the followingformula (1).

$\begin{matrix}{D = {\frac{C \cdot H^{2}}{1 + \sqrt{1 - {\left( {1 + K} \right) \cdot C^{2} \cdot H^{2}}}} + {E_{4} \cdot H^{4}} + {E_{6} \cdot H^{6}} + {E_{8} \cdot H^{8}} + {E_{10} \cdot H^{10}} + \ldots}} & (1)\end{matrix}$

FIGS. 22A and 22B are tables illustrating coefficients for forming areflection plane of the second mirror 10.

The reflection plane of the second mirror 10 is computed by theapplication of the coefficients illustrated in the tables of FIGS. 22Aand 22B based on the following formula (2).

$z = {\frac{{cr}^{2}}{1 + {{SQRT}\left\lbrack {1 - {\left( {1 + k} \right)c^{2}r^{2}}} \right\rbrack}} + {\sum\limits_{j = 2}^{72}\;{C_{j}x^{m}y^{n}}}}$where

-   z represents a sagittal height of a plane parallel to z axis,-   c represents a vertex curvature (CUY),-   k represents a conic constant, and-   Cj represents a coefficient of monomial expression x^(m)y^(n)-   (2)

FIG. 23 is a table illustrating a layout of the first mirror 9, thesecond mirror 10 and the dustproof glass 11.

With the projector 100 having the aforementioned configuration, it maybe possible to provide an image display apparatus capable of displayingan enlarged image on the screen by projecting light from a positionextremely close to the screen.

Note that “**” in the table of FIGS. 22A and 22B representsexponentiation. Note also that “*” in the table of FIGS. 22A and 22Brepresents multiplication.

As described above, the image display apparatus according to theembodiments may be capable of displaying an enlarged image on the screenat a close range by adjusting the converting position of the projectionluminous flux exhibiting high divergence.

Further, the image display apparatus according to the embodiments may becapable of displaying an enlarged image on the screen at a close rangewhile reducing an adverse effect of dust reflecting on the screen byadjusting the image forming position of the intermediate image.

According to the aforementioned embodiments, it may be possible toprovide an image display apparatus capable of projecting light todisplay an enlarged image on the screen even if the image displayapparatus is placed at a position extremely close to the screen.

According to the aforementioned embodiments, it may be possible toprovide a small-sized image display apparatus capable of projectinglight to display an enlarged image on the screen at a close range whilereducing an adverse effect of dust reflecting on the projected image onthe screen even if the dust is attached to the lens optical system.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the principlesof the invention and the concepts contributed by the inventor tofurthering the art, and are to be construed as being without limitationto such specifically recited examples and conditions, nor does theorganization of such examples in the specification relate to a showingof the superiority or inferiority of the invention. Although theembodiment of the present invention has been described in detail, itshould be understood that various changes, substitutions, andalterations could be made hereto without departing from the spirit andscope of the invention.

What is claimed is:
 1. A projection optical system for an image displayapparatus including an image display device and configured to project aprojection image formed by the image display device onto a projectionsurface, comprising: a refractive optical lens system having a positiverefractive power and including a plurality of lenses; a first mirror;and a second mirror formed of a concave mirror, wherein the refractiveoptical lens system includes one optical axis shared by all lenses ofthe plurality of lenses, and the optical axis is parallel to theprojection surface, a lens surface located closest to the first mirroris convex, and a first light beam which reaches a position closest to animage display surface of the image display device on the projectionsurface and a second light beam which reaches a position farthest fromthe image display surface of the image display device on the projectionsurface are reflected by the second mirror, and then intersect at aposition closer to the first mirror than the second mirror in a planeincluding the optical axis and perpendicular to the projection surface.2. The projection optical system as claimed in claim 1, wherein thefirst mirror is a plane mirror.
 3. The projection optical system asclaimed in claim 1, wherein a position at which the first light beam andthe second light beam intersect is adjacent to a virtual reflectionplane extended from a reflection plane of the first mirror.
 4. An imagedisplay apparatus comprising: the projection optical system as claimedin claim 1; an illumination optical system configured to apply lightemitted from a light source; and the image display device configured toreceive the light applied from the illumination optical system to formthe projection image.
 5. The image display apparatus as claimed in claim4, wherein each of angles formed between rays of light incident on animage projected onto the projection surface at a center in a horizontaldirection and a normal line of the projection surface has a same sign.6. The image display apparatus as claimed in claim 4, wherein an end ofone of the lenses of the refractive optical lens system located closestto the first mirror is protruded more toward a first mirror side than aside end of a lens barrel holding the refractive optical lens system. 7.The image display apparatus as claimed in claim 4, wherein one of thelenses of the refractive optical lens system located closest to thefirst mirror is an atypical lens partially having a cutout part.
 8. Theimage display apparatus as claimed in claim 4, wherein the image displaydevice is a reflection image display device that includes a plurality oftwo-dimensionally arranged micromirrors, and wherein emission ofreflected light is switched on or off by changing an angle of each ofthe micromirrors to be in an ON state or an OFF state.
 9. A projectionoptical system for an image display apparatus including an image displaydevice and configured to project a projection image formed by the imagedisplay device onto a projection surface, comprising: a refractiveoptical lens system having a positive refractive power and including aplurality of lenses; a first mirror; and a second mirror formed of aconcave mirror, wherein the refractive optical lens system includes oneoptical axis shared by all lenses of the plurality of lenses, and theoptical axis is parallel to the projection surface, a lens surfacelocated closest to the first mirror is convex, and a first light beamwhich is corresponding to a pixel closest to the optical axis in a planeincluding the optical axis and perpendicular to the projection surfaceand a second light beam which is corresponding to a pixel farthest fromthe optical axis in the plane are reflected by the second mirror, andthen intersect at a position closer to the first mirror than the secondmirror in the plane.
 10. The projection optical system as claimed inclaim 9, wherein the first mirror is a plane mirror.
 11. The projectionoptical system as claimed in claim 9, wherein a position at which thefirst light beam and the second light beam intersect is adjacent to avirtual reflection plane extended from a reflection plane of the firstmirror.
 12. An image display apparatus comprising: the projectionoptical system as claimed in claim 9; an illumination optical systemconfigured to apply light emitted from a light source; and the imagedisplay device configured to receive the light applied from theillumination optical system to form the projection image.
 13. The imagedisplay apparatus as claimed in claim 12, wherein each of angles formedbetween rays of light incident on an image projected onto the projectionsurface at a center in a horizontal direction and a normal line of theprojection surface has a same sign.
 14. The image display apparatus asclaimed in claim 13, wherein an end of one of the lenses of therefractive optical lens system located closest to the first mirror isprotruded more toward a first mirror side than a side end of a lensbarrel holding the refractive optical lens system.
 15. The image displayapparatus as claimed in claim 13, wherein one of the lenses of therefractive optical lens system located closest to the first mirror is anatypical lens partially having a cutout part.
 16. The image displayapparatus as claimed in claim 13, wherein the image display device is areflection image display device that includes a plurality oftwo-dimensionally arranged micromirrors, and wherein emission ofreflection light is switched on or off by changing an angle of each ofthe micro mirrors to be in an ON state or an OFF state.